Here are some detailed thoughts with photos, links, etc.
First, here's how the Pagoda 5500G looks on my wrist:
It's a nice watch in itself, but I am not too fond of how it wears.
Another reference with a shape belonging to the same "family" is the
1593 "Hourglass" , but, again, I didn't like it on my wrist. The photos are in this dedicated thread:
www.watchprosite.com
And also here, with an incredible pink on pink example. In this thread, you will also see a
ref. 425 , which I'm also very fond of (it is close to the 2461 which I discuss below):
www.watchprosite.com
And let me just add these two photos from Hodinkee, which show how great (imo) this ref. can look on a wrist:
credit: Hodinkee
Concerning the
ref. 492 , I have read on
Sotheby's website that only 150 pieces in were made in total.
You can see this reference inside the thread dedicated to
ref. 5105P , a grail I would like to own):
www.watchprosite.com
The ref. 2461 is a reference I love because the rectangular shape and the 22 x 42 mm case looks great on the wrist. Ref. 425, featured above with the 1450 Top Hat is a little smaller: 20 x 40 mm.
Some examples also feature a faceted crystal, which I like a lot. And, of course, it is powered by legendary manual-winding shaped caliber 9-90, which I'm highly attracted to.
Another reference you might want to consider, even though it is very rare, is the
ref. 2531 :
www.watchprosite.com
I loved it when I tried it on, but the example didn't have a correct buckle, came without papers and was, imo, a little overpriced.
I haven't seen it in real life, but I think I could like it.
My hands-on experience with the
square ref. 3404 (and overview of different examples) is here:
www.watchprosite.com
Definitely a very nice timepiece, but the round caliber 23-300 attracts me less than the rectangular 9-90, which is the caliber I would really love to add to my collection.
Finally, regarding the square shape, I just want to add a few words about the modern
5489 "Trapeze" . Not only is it too modern for what you're looking for (it dates back to the 2000s), but also, while I found the photos very exciting (my threads are
here and also
here and
here), my experience in hand was very disappointing:
My thread dedicated to the
ref. 2470 (my personal favortie) and the
ref. 2469 is here:
www.watchprosite.com
I like the 2470 very much (shape, 9-90 caliber, indices on the dial), but it is extremely rare and I doubt that there are some white metal examples...
Ref. 2554 "Manta Ray" is another reference you might want to pat attention too:
www.watchprosite.com
My personal hands-on experience was disappointing because the watch looked a little too bland or dull:


But on these photos from
Hodinkee, I find it
attractive:

credit: Hodinkee
Other references you could be considering are:
As a conclusion, here's what I would say:
Your gorgeous 2509R with Type 1 dial is powered by the wonderful round manual-winding caliber 12-400.
Therefore, imo, the best way to complement it and create a magnificent two-piece collection is to choose a reference with the shaped caliber 9-90, the only one that truly rivals your 12-400.
Which reference?
That can only be a matter of personal taste and preference.
As you've probably guessed, I'm very attracted to ref. 492 and 2461, but I would also love a 1531 and I'm also very intrigued and attracted to the 1450 "Top Hat".
In any case, I hope this detailed answer will be helpful and I wish you a wonderful quest, hoping that you find the reference that fully resonates with you.
Best cordial regards,
Emmanuel